I keep meaning to bring this up but always forget. I was told, by a car dealer, in reference to my A/T, that I should always drive around town in "3" and only ever use "D" for highway driving. He said this is the best way to conserve fuel and get the best MPG.

PS> I have tried both for several tanks of gas in my circumstance trying to figure out what is wrong with my Yaris and its very poor MPG. I have noticed no difference, but this may be due to something being wrong with my Yaris.

Myth. Even in city driving, my Yaris goes into OD. For best gas mileage, you want it in as high as gear as possible. I only drop it into '3' when I need some extra power such as going up steep hills...

...and drive, Drive, DRIVE! - It will work automatically. If you go up a steeper hill, it will most likely drop down a gear or two. I live in the Appalachians in Western North Carolina, when I come down the mountain it drops down & slows the car automatically. Have driven a lot of vehicles that I had to lay on the brakes to slow down, not this one. This is a sweet little transmission...just put it in drive and GO

The only advantage to running it in 3 is in 45 and under hilly areas where it would shift into and out of OD real often.It is more of a nuisance thing than anything else.....the constant downshifting on a hill. Other than that, just keep it in D for DRIVE!!!

I'm quite impressed by how smart my 2013 Yaris automatic transmission is - if I'm going downhill, and leave my foot really, really lightly on the accelerator, the fuel consumption drops to less than 1.0 l/100km, if I take my foot completely off the accelerator above 70km/h, the consumption drops to 0, and the car gradually slows under engine braking,

What I've recently noticed is that under 70km/h, if I take my foot off the accelerator completely, slight engine braking happens, with low (but not zero) fuel consumption (as if the car knows I want it to keep rolling).

But, if I tap the brake pedal, the fuel consumption drops to 0, and more significant engine braking happens. So, if I'm approaching red lights, I lift off the accelerator, tap the brake, and coast to a stop using zero fuel, whereas if I *don't* tap the brakes, I still coast to a stop, but more gradually, and using (slightly) more fuel.

These behaviours don't seem to be documented anywhere in the owners' manual.

3 prevents the AT from going into top gear, which would lower the rpms. High rpms produce more horse power, but uses more gas.

No real harm in staying in 3, but it does use more fuel, and there is more engine wear. How much? Don't know. My Kia would be at 4K RPM's at 80 mph, and that engine seemed to be still in perfect condition at 140K miles, that engine was turning fast all the time.

I keep meaning to bring this up but always forget. I was told, by a car dealer, in reference to my A/T, that I should always drive around town in "3" and only ever use "D" for highway driving. He said this is the best way to conserve fuel and get the best MPG.

I'm quite impressed by how smart my 2013 Yaris automatic transmission is - if I'm going downhill, and leave my foot really, really lightly on the accelerator, the fuel consumption drops to less than 1.0 l/100km, if I take my foot completely off the accelerator above 70km/h, the consumption drops to 0, and the car gradually slows under engine braking,

What I've recently noticed is that under 70km/h, if I take my foot off the accelerator completely, slight engine braking happens, with low (but not zero) fuel consumption (as if the car knows I want it to keep rolling).

But, if I tap the brake pedal, the fuel consumption drops to 0, and more significant engine braking happens. So, if I'm approaching red lights, I lift off the accelerator, tap the brake, and coast to a stop using zero fuel, whereas if I *don't* tap the brakes, I still coast to a stop, but more gradually, and using (slightly) more fuel.

These behaviours don't seem to be documented anywhere in the owners' manual.

Same manual also states to only use 3/2/L for engine braking or for short period of powering the wheels, but not for extended use (aka regular driving). My understanding of the statements of the user manual so far is that 3/2/L gears are there as an option to take advantage of more controlled engine braking and for emergency manual gear selection, in case the transmission goes bad, you can still manage to drive to your destination/safe zone/repair shop with a lower gear that won't strain the damaged drivetrain too much. I don't think as of now that you should select any of the 3/2/L instead of OD for driving, as it would be the equivalent of driving a manual transmission in a single gear with the engine struggling for staying in the ideal RPM range non-stop. That's how I understand it for now.

It really depends on what you want. Leaving the transmission in D will allow it to select the most economic gear for the speed you are travelling. Putting it in 3 will lock out the overdrive 4th gear, providing slightly quicker acceleration and more engine braking while using more fuel.